What causes a mirage? –Georgia, 10, Wash.

Imagine you’re riding in the car on a very hot day, when you look out the window and see a shimmering puddle of water up ahead. As you get closer, you find there’s not really anything there. It’s a mirage.

While you aren’t really seeing a puddle, you are definitely seeing one of light’s many wonders. It has to do with the way light travels, and with the way our eyes and brain take in light.

That’s what I found out when I went to visit my friend Jeff McMahon, a physicist at Washington State University.

If you’ve ever tried to get some place in a hurry, you probably took the fastest way possible. Light does this, too. It travels fast and wants to take the shortest path, McMahon said. But sometimes it will slow down if something gets in its way. Cold air, for example. Light can travel faster through hot air than cold air.

Now think of a paved road on a hot day. The ground would be really hot and so would the layer of air right above it. Meanwhile, any air above this warm layer would get colder and colder.

Light will travel slowly through the cold air. However, if light wants to take the shortest and fastest trip, it will swoop down into the warm air near the ground and speed up.

But the light still has to go back up to your eyes so you can see. The light heads back up toward you, traveling through the colder air again. It makes a bit of a U-shaped trip. And where the cold and warm air layers meet, the light bends.

Your eyes and brain try to figure out what’s going on. After all, they are used to taking in light that comes in at a straight line. When the light bends, your brain and eyes see a mirage.

This bending light is what we call refraction. Where the light bends, you’ll likely see an image of the sky refracted on the ground.

This doesn’t just happen on pavement, McMahon said. It can also happen on water. People have seen mirages of boats and islands. Some people think this is where we might have come up with the stories of ghost ships. Little did they know at the time it was all refraction—a journey of light. Can you think of how refraction might happen on the ocean?

Refraction happens in lots of places in our universe. Here’s an easy way to really surprise your friends and classmates, while also seeing how light can work when it passes through different materials. Draw an arrow on a piece of paper. Then, place the paper behind a glass. Fill the glass with water. What happens to the arrow? Tell me about it sometime at Dr.Universe@wsu.edu.

Sincerely,
Dr. Universe

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